South Korea's militant umbrella union on Thursday held a rally in Seoul in a one-day walkout in a prelude to an indefinite general strike, calling for an end to layoffs and regularization of temporary workers.

Some 40,000 unionized workers, including truckers and construction workers already on strike, attended the rally organized the Korean Confederation of Trade Unions (KCTU).

The country's second largest trade union with 580,000 members warned it will go on an indefinite strike starting August 18 unless the government takes steps to revise labor laws to meet its demands.

The metal workers' union and the financial industry union also plan a strike next month.

Unionized truckers, disgruntled with rising fuel prices and low freight charges, are already on an indefinite strike that started earlier this week.

Separately, unionized construction workers are also staging a walkout, demanding better welfare benefits and protesting unpaid wages.